The invention relates generally to a method and apparatus for the dissolution and transport of a imaging agent, for use in magnetic resonance imaging (MRI) and analytical high-resolution NMR spectroscopy.
MRI is a diagnostic technique that has become particularly attractive to physicians as it is non-invasive and does not involve exposing the patient under study to X-rays associated with other medical imaging techniques. Analytical high resolution NMR spectroscopy is routinely used in the determination of molecular structure. MRI and NMR spectroscopy can, however, lack sensitivity due to the normally very low polarization of the nuclear spins of the contrast agents typically used. As such, a number of techniques exist to improve the polarization of nuclear spins while in the solid phase. These techniques are known as hyperpolarization techniques and lead to an increase in sensitivity. In hyperpolarization techniques, a sample of an imaging agent, for example 13C-pyruvate or another similar imaging agent capable of being polarized, is introduced or injected into the subject being imaged. The hyperpolarized 13C-pyruvate is obtained from dynamic nuclear polarization (DNP) of 13C-pyruvic acid using an Electron Paramagnetic Agent (EPA).
In many instances, the imaging agent undergoes this hyperpolarization in an apparatus remote from its end use. The hyperpolarization has a very short life span, and as such, the hyperpolarized material must be quickly transformed into a useable state and transferred from its production source to its place of intended end use (i.e., injection into a patient). To accomplish this, the cryogenically frozen hyperpolarized material is dissolved into a dissolution medium for injection into the patient. Thus, as a part of a dynamic nuclear polarization (DNP) system, a means for dissolving the polarized sample within the polarizer must be included.
In the current methodology, for a sample of polarized acid (e.g., pyruvic acid), it is necessary to use a dissolution medium to dissolve the sample and obtain a solution of polarized sodium salt (e.g., sodium pyruvate) suitable for in vivo injection. The dissolution medium (DM) typically is comprised of an alkaline solution including a base (e.g., sodium hydroxide) and a buffering agent (e.g., TRIS hydroxymethyl aminomethane (TRIS)) to dissolve and neutralize the sample.
In this process a defined volume of dissolution medium containing sodium hydroxide, TRIS-buffer, and EDTA (ethylenediaminetetraacetic acid) is pressurized within a plastic cylinder and heated to a defined temperature. When the dissolution process is started, the pressurized and heated solvent is released from the cylinder and guided by a fluid path into contact with the polarized sample. The dissolved sample is ejected through a filter, which chemically retains the EPA. The dissolved polarized sample passes to a receiver vessel. The receiver vessel may be empty, contain additional dissolution medium, or water for dilution or temperature regulation.
The basic DM produces a dyanmic pH profile during the dissolution so it interacts with acidic hyperpolarized imaging agent 13C-pyruvic acid (13CPA) and EPA mixture. Problems can arise from the dynamic pH profile of the dissolution as it is a major factor in a successful dissolution. It can influence, EPA filtration, liquid-state polarization (LSP), final pH, and pyruvic acid concentration in the solution. The dynamic pH profile actually enables EPA filtration with specific water wettable resins. Also the final pH of the solution to be injected requires the correct amount of sample and DM to be flushed out of the system and can therefore be influenced by the dynamic pH profile. The profile, the basic nature of the DM, significant temperature, and other chemical burdens necessitate a robust material to prevent degradation.
With respect to EPA filtration, the system requirements have been addressed by three previous specific methods: pre-mixing the solution before filtration, using a pre-conditioned filter, or using an unconditioned filter. The pre-mixing method allows all of the components of the solution to collect in a vessel where they are mixed and neutralized after the dissolution. This in effect bypasses the need to consider the dynamic pH profile. After the solution is mixed, it is then pushed through a filter to remove the EPA and collected in a second vessel where it is checked for quality before being dispensed to the patient. This method would require two collection vessels, two sources of pressure—one at the syringe and one at the first receiver to push the solution through the EPA filter—to cause the fluid to flow and would take more time than the following methods. The total time of the dissolution is important because the hyperpolarized parenteral is only active for a short time.
Compared to the pre-mixing method, making use of either a pre-conditioned filter or an unconditioned filter eliminates the need for two collection vessels and two sources of pressure now requiring only one in each instance. It also reduces the overall time for the dissolution.
The issue with the pre-conditioned filter is that the pre-conditioning step uses a solvent to activate the filter and this activation needs to be preserved from activation in the factory to the point of final use. Care must be taken that the solvent used for activation is not injected into the patient unless it has been determined that it is an acceptable solvent for injection. An acceptable solvent for injection may not always be the best solvent for activation. One way to preserve the activation would be to freeze the filter. This would require that the filter be stored frozen and shipped frozen to its location of use and thawed out prior to use. These preservation steps may be costly and the actual temperature of the filter would have to be monitored to know whether or not the filter warmed up at any point along the way allowing for deactivation or possible biological contamination.
An unconditioned filter removes the need for the solvent activation and preservation of the activation however; the resin used in this filter is typically expensive due to its unique chemical structure and properties. Further the hyperpolarized pyruvic acid appears to more strongly interact with the water wettable resins than the neutral sodium pyruvate. For this reason it is desirable to make the transition to sodium pyruvate as quickly as possible when using the water wettable resins in the basic dissolution. The need for a rapid pH change means that significant heat must be introduced in the form of high dissolution media temperature. The alkaline dissolution process therefore requires rapid acid neutralization to minimize polarization loss due to filter resin interaction. The rapid neutralization may limit the potential for low temperature dissolutions.
Thus, a need therefore exists for a method and apparatus that can reduce the problems that can arise from using an alkaline dissolution process.